Krypton and xenon are undergoing increasing demand in a number of applications. Krypton is being widely used in high quality lighting including long-life light bulbs and automotive lamps. Xenon is being used for medical applications including special x-ray equipment. Both of these gases are commonly used in many laboratory and research applications.
The principle source of krypton and xenon is the atmosphere. Atmospheric air contains about 1.1 ppm (parts per million) of krypton and about 0.08 ppm of xenon. Generally, krypton and xenon are recovered from air in conjunction with a comprehensive air separation process which separates air into oxygen and nitrogen.
At the heart of krypton and xenon recovery processes is the fact that krypton and xenon have lower vapor pressures than the major atmospheric gases. This allows their concentration, in vapor-liquid countercurrent distillation processes, to increase to the point where recovery is economically viable. The krypton and xenon concentrate in the oxygen component rather than the nitrogen component because oxygen has a lower vapor pressure than nitrogen. Unfortunately these processes also unavoidably concentrate atmospheric hydrocarbons which are also characterized by lower vapor pressures than the major atmospheric gases, thus giving rise to an increased danger of explosion.
A recent attempt to address this problem is disclosed in U.S. Pat. No. 4,401,448-LaClair, wherein the oxygen component in the krypton-xenon concentrate is replaced by non-combustible nitrogen thus markedly reducing dangerous conditions. Although the process disclosed in the aforementioned patent successfully exchanges oxygen with nitrogen in the rare gas concentrate, it does not recover all of the krypton and xenon in the original oxygen-containing concentrate, thus requiring either a loss of some of the rare gas or, alternatively, a return of the waste stream to an air separation plant for further processing to recover the krypton and xenon. This alternative is undesirable for two reasons. First, krypton and xenon, which had already been concentrated must be remixed with the fluids in the air separation plant and again undergo rectification, resulting in added costs. Second, the krypton-xenon concentration process must necessarily be close to, and operated in conjunction with, the comprehensive air separation plant, resulting in loss of flexibility and possibly higher costs.
Accordingly, it is an object of this invention to provide an improved process to produce a krypton-xenon concentrate in an oxygen-free medium.
It is another object of this invention to provide an improved process to produce a krypton-xenon concentrate in an oxygen-free medium wherein substantially all of the krypton and xenon which enters the process is recovered in the concentrate.
It is a further object of this invention to provide an improved process to produce a krypton-xenon concentrate in an oxygen-free medium which can be operated economically independently of a cryogenic air separation plant.